Equally, until very recently, the lack of a human trophoblast stem cell restricted in vitro experiments on trophoblast proliferation and invasion to the use of choriocarcinoma or trophoblast-like cell lines that are karyotypically abnormal or often a [isshp.org]

The secretions also contain a variety of growth factors that when applied exogenously to placental explants stimulate proliferation of the trophoblast cells 3. [isshp.org]